1. Field of the Invention
The present invention relates to geophones, namely devices which can be used to detect sonic or infrasonic vibrations, especially from the earth's crust. It relates more particularly to geophones with a sensitive element made of a piezoelectric polymer.
2. Description of the Prior Art
According to a known method, for example one described in the U.S. Pat. No. 4,144,520, a geophone is made by using a permanent magnet forming an inertial mass, suspended in a casing, which is itself in contact with the medium subjected to the vibrations that have to be detected, which is most usually the ground. A solenoid coil is wound around the casing in such a way that the movements of the magnet within the casing induce electric voltage in this solenoid coil. These movements are due to the inertia of the magnet which remains subtantially still when the casing vibrates through the movement of the ground because of an earthquake for example.
There are known polymer or copolymer materials which, when suitably processed, display piezoelectric properties. The most widely known material is polyvinylidene fluoride, abbreviated as PVF.sub.2.
A known method uses a two-strip sensor, such as the one shown in FIG. 1, using two superimposed strips 101 and 102, separated by a conducting electrode 103. These strips are polarized in the same direction, and their external surfaces are covered with conducting electrodes 104 and 105. This two-strip sensor is embedded in a part 106 which, in this case, also provides for connection with the external electrodes 104 and 105, and with an output connection 107. The central electrode 103 is itself joined to a second output connection 108.
When this two-strip sensor is subjected, as in the figure, to a movement of deflection with respect to the embedding part 106, an electrical signal appears between the connections 107 and 108, and this electrical signal increases with the deflection of the two-strip sensor.
An alternative embodiment consists in using two layers of PVF.sub.2, polarized in opposite directions, and in picking up the output signal between the external electrodes.
There are also known methods of making piezoelectric polymer based microphone sensors, as described for example in the French Pat. No. 2 542 552 by the Applicant. It is certain that, by fixing a microphone of this type to the ground, a small signal, due to the movement of the ground, is obtained on the microphone. However, the sensitivity of a system of this type is quite poor because there is no inertial mass, and the resonance frequency is far too high to enable the accurate detection of infrasonic vibrations.